A magnetic-inductive flowmeter comprises a tubular measured-value sensor which is attached between the connecting flanges of a pipeline and through which flows a conductive flow transverse to the direction of a magnetic field. A voltage proportional to the flow rate is tapped off at two measuring electrodes and conducted via a current conductor to a signal converter. Usually the measuring tube of the measured-value sensor consists of a metal tube provided with an inner jacket with an insulating layer or a plastic tube.
The U.S. Pat. No. 3 750 468 describes a construction with a titanium measuring tube which is fitted over an insulating alumina tube by shrink-fitting. The metallic electrodes pass through an insulating bushing fitted in holes bored in the tube. To effect a fluid-tight seal between the head of the electrodes and the wall of the bore in the alumina tube the alumina wall is metallized and the conical edge of each head is welded thereto. The German Patent application No. 29 50 039 describes a measuring tube with electrodes made of particles of carbon, graphite or metal in the form of powder or fibers embedded in the insulating liner of the measuring tube. Measured-value sensors consisting of a metallic tube providing the mechanical strength and having an insulating liner at the inner wall are complicated and expensive.
On the other hand, a ceramic measuring tube is well known having the advantage of being electrically insulating and resistant to corrosion and which is constructed as a self-supporting member. The German Patent application DE-AS No. 23 30 593 describes a measured-value sensor with a ceramic measuring tube which, with the interposition of sealing rings, can be clamped between two flanges of a pipeline. On the ceramic electrically insulating tube the electrodes are fused to the vitrified inner jacket and are provided with conducting wires which pass through radial bores. The flanges are provided with an annular shoulder against which abuts a sheet metal shroud wrapped from the outside. This construction has the disadvantage that the radial bores form weakness zones in the ceramic measuring tube which, when fluids flow through under high pressure, can result in cracks and destruction of the ceramic body. Furthermore, the fusion of the electrodes onto the vitrified inner jacket during firing does not ensure an absolutely tight seal. The glazed coating on the ceramic body also cracks frequently because of different thermal expansions and stresses, so that the fluid under high pressure can penetrate through pores of the body to the outside.
Furthermore, the ceramic measuring tube of the measured-value sensor is highly susceptible to the effects of impacts and shocks and is able to absorb tensile stresses to a limited extent only. Such ceramic masses likewise do not offer good resistance to changes in temperature such as can occur in conjunction with the hot fluids that have to be measured. The measured-value sensor of the German Patent application DE-AS No. 23 30 593 is therefore not suitable for fluids under high pressure and aggressive and hot fluids.
The German Patent application DE-B No. 10 98 727 has a ceramic measuring tube with measuring electrodes which are designed in the form of a hollow cylinder with a reinforced bottom. The electrodes are fused into radial bores of the measuring tube with a vitreous mass having a coefficient of expansion which is between the coefficients of the materials of the electrodes and the ceramic tube. But a good permanent seal is not available and the vitreous mass cracks because of changes in temperature and of the effects of impacts and shocks. The vitreous mass is not resistant to many mediums.
The European Patent application EP-A1 No. 0047342 describes a measured-value sensor of plastic or ceramic, which can be inserted in a tubular steel casing and with magnetic coils inserted from the outside into recesses passing radially through the casing, and projecting into blind holes of the measuring tube. The measuring electrodes are inserted in lateral bores of the measuring tube and sealed. This construction is, however, because of the arrangement of lateral recesses and bores practically suitable only for measuring tubes of plastic material but not for those of ceramic materials.
In the common field of electrodes the German Patent application DE-A No. 21 19 705 proposes to attach a coating of a mass which includes noble metals or alloys of noble metals to the surface of the electrodes by burning in order to keep the resistance of polarization and the capacitive effect low at the surface of the electrodes. This proposal is concerned only with the coating of a usual metallic electrode for optimizing the electric properties of the electrodes.
For measured-value sensors it is also known to mount between the measuring tube and the connecting flanges of the pipeline, grounding rings that are in contact with the conductive fluid and which are directly grounded or conductively connected with the grounded metal casing of the measured-value sensor. These grounding rings require sealing rings on both sides. Such a construction is complicated and frequently leads to leakages.